1. Field of the Invention
The present invention relates in general to sensors and, more particularly, to a fiber-optic pressure sensor for use in hostile environments and methods of packaging the pressure sensor.
2. Description of the Related Art
In many engineering applications an accurate determination of both static and dynamic pressures is needed for optimized performance as well as the early detection of undesirable operating conditions. Consider, as an example, but not a limitation, the fact that in gas turbine and/or aircraft engines, overall efficiency is directly related to the maximum pressure in the cycle and compressor performance in such engines may be affected by sudden changes in pressure, particular during off-design performance. In internal combustion engines, engine knock and misfire are two undesirable phenomena directly related to pressure inside the cylinder of the engine where pressure measurements may be directly related to the overall engine performance. As such, the ability to monitor continuously pressure fluctuations may significantly improve engine efficiency, performance, reliability and operating costs as well as permit lean-burn engine operations, a wider tolerance to fuel octane, and acceptance of alternative fuels. Harsh environments characterize the operating conditions in these engines, among many other applications, where corrosive conditions, elevated temperatures, and electromagnetic interference, or EMI, make it difficult for the proper use of electrical pressure probes, such as piezoelectric sensors. This is so because piezoelectric pressure transducers are limited due to signal variation caused by temperature and electromagnetic interference and dynamic response limitations caused by the need for the use of an intermediate interface between the environment and the sensing element. Furthermore, signal amplification electronics cannot be located near the sensing element in these harsh environments.
An optical interface between the sensor and a signal conditioner is more robust in the above-mentioned harsh environments and does not require closely coupled electronics that have high-temperature limitations. However, one of the challenges of making a high-temperature optical dynamic pressure sensor is the development of an assembly, or packaging, that can survive elevated temperatures and minimally affect the sensor output. One example of such optical devices is a Fabry-Perot interferometer, which is a fiber optic sensor sensitive to pressure or stress in a manner that causes a beam of light to be reflectively modulated in response to changes in pressure or stress on the sensor. The spectral response of an optical interferometer is a periodic function having a sinusoidal shape, the period of which is inversely proportional to differences in the optical path of the sensor. Thus, measured changes in light modulation are measured and correlated with changes in flow variables of interest, such as, for example, pressure and temperature.
Optical interferometers are known devices that have been used to detect a variety of physical parameters, as shown, for example, in U.S. Pat. No. 4,360,272 (Schmadel et al.), U.S. Pat. No. 4,714,342 (Jackson et al.), U.S. Pat. No. 4,942,767 (Haritonidis et al.), U.S. Pat. No. 4,688,940 (Sommargren et al.), U.S. Pat. No. 5,179,424 (Lequime et al.), U.S. Pat. No. 5,200,796 (Lequime), U.S. Pat. No. 5,202,939 (Belleville et al.), U.S. Pat. No. 5,206,924 (Kersey), U.S. Pat. No. 5,349,439 (Graindorge et al.), U.S. Pat. No. 5,619,046 (Engstrom et al.), U.S. Pat. No. 6,122,971 (Wlodarczyk), and U.S. Pat. No. 6,842,254 (Van Neste et al.).
However, the application of optical interferometers to high-temperature, harsh environments has been limited due to difficulties associated with packaging and reliability. For example, due to variations in coefficient of thermal expansion of the different materials used, the stress state of optical cavities in conventional interferometers are significantly affected by temperature, causing unwanted changes in optical cavity dimensions and a high level of measurement uncertainty. In addition, because of the difficulty in eliminating the relative motion of the fiber optic with respect to the optical cavity, additional undesired spectral signals are generated in the gaps between the fiber optic cable and the cavity, thus generating noise in the measured signal. Furthermore, given the high-temperature environment in the industrial applications sought herein, conventional devices are made of materials that would simply not survive in environments in which the average temperature of the sensor may exceed 350° C. and peak temperatures may be much higher. Furthermore, high temperature accelerates oxidation and corrosion of the packaging materials in the presence of contaminates from the environment and combustion by-products, such as sulfur, thus limiting the reliable functioning of the sensor, and diffusion of metals that lead to brittle intermetallic compounds and failure.
Therefore, based at least on the foregoing summarized discussion, a need exist for a fiber optical pressure sensor with simple optics capable of reliable operation in high-temperature environments. Among other advantageous features, the fiber optical pressure sensor and associated packaging method disclosed herein provide a sensor that is small and light weight so as to allow accessibility and use in harsh environments with high levels of temperature and heat radiation, passive (i.e., non-electrical) data collection, and high sensitivity and frequency response.